1. Field of the Invention
The present invention relates to a method of obtaining a crystalline semiconductor used for thin film devices such as a thin film insulated gate type field effect transistor (thin film transistor or TFT).
2. Description of the Related Art
Conventionally, a crystalline semiconductor thin film used for thin film devices such as a thin film insulated gate type field effect transistor (TFT) has been fabricated by crystallizing an amorphous silicon film formed by a plasma CVD method or thermal CVD method at a temperature of more than 600xc2x0 C. in an apparatus such as an electric furnace.
This conventional method, however, has had various problems. The biggest problem has been that it is difficult to obtain a good product because the crystalline silicon film obtained is polycrystal and there is difficultly in controlling its grain boundary, and its reliability and yield is not so high because of its dispersion characteristic. That is, because the silicon crystals obtained by conventional heat treatment grow totally at random, it has been almost impossible to control the direction of its crystal growth.
Accordingly, it is an object of the present invention to solve the aforementioned problems by providing a method for controlling the growth of the crystal.
According to the present invention, crystal growth is controlled, and a TFT having high reliability and high yield is obtained, by forming a gate electrode on a silicon film in an amorphous state or in a random crystal state (for example a state in which portions having good crystallinity and amorphous portions exist in a mixed state) which can be said to be a substantially amorphous state, by forming impurity regions within the silicon film using the gate electrode as a mask, forming regions containing at least one of nickel, iron, cobalt platinum or palladium so that they adhere on part of the impurity regions, and by annealing the whole to crystallize it starting from the region containing nickel.
In particular, the present invention allows substantial elimination of the grain boundary between the source and drain and the active layer and to obtain a good characteristic by advancing the crystallization of the source and drain at the same time as the crystallization of the active layer (channel forming region).
A method of growing a crystal of silicon film epitaxially in solid phase centering on a crystalline island film as a nucleus or as a seed crystal has been proposed as a prior art method (for example, Japanese Patent Laid-Open No. 1-214110, etc.). However, it has been difficult to suppress crystal growth from other sites even if the crystal nucleus exists. That is, because the annealing temperature for growing the crystal is a temperature suited for fully generating the crystal nucleus, the crystal often starts to grow from unexpected locations.
The inventor of the present invention found that nickel (Ni), cobalt, iron platinum and palladium are readily coupled with silicon and that the crystal would grow centering on them. The inventor noticed that nickel is readily made into nickel silicide (NiSX, 0.4xe2x89xa6xc3x97xe2x89xa62.5) and that the lattice constant of the nickel silicide is close to that of silicon crystal, then devised a method of growing a silicon crystal centering on the nickel silicide. Actually, the crystal growing temperature could be lowered by 20xc2x0 C. to 150xc2x0 C. compared to that of the conventional method. Because no crystal nucleus was produced in a pure silicon film at this temperature, crystals did not grow from unexpected locations. It was assumed that the crystal grew from the crystal nucleus by the same mechanism as the conventional one and that the higher the temperature, the faster the speed of advancement of the crystallization, at temperatures at which crystal nuclei did not grow naturally (preferably less than 580xc2x0 C.). A similar effect was seen also with iron (Fe), cobalt (Co) platinum (Pt) and palladium (Pd).
According to the present invention, a film or the like containing a simple substance of nickel, iron, cobalt, platinum or palladium or their suicides, acetates, nitrates and other organic acid salts is adhered to the impurity regions of the thin film transistor, and the region of the crystal silicon is expanded away therefrom as the starting point. By the way, oxide is not preferable as the material containing the aforementioned material because oxide is a stable compound and a silicide which is likely to become the crystal nucleus is not produced therefrom.
The crystal silicon which expands thus from a specific location has a structure close to a monocrystal having good continuous crystallinity. A better result could be obtained with an amorphous silicon film having less hydrogen concentration serving as the starting material for crystallization. However, because hydrogen was released as crystallization advanced, no clear correlation could be seen between the hydrogen concentration within the silicon film obtained and that of the amorphous silicon film as the starting material. The hydrogen concentration within the crystal silicon of the present invention was typically more than 1xc3x971015 atomsxc2x7cmxe2x88x923 0.01 atomic % and less than 5 atomic %.
While a heavy metal material such as nickel, iron, cobalt or platinum or palladium is used in the present invention, those material themselves are not suitable for silicon as a semi-conductor material. It is therefore necessary to remove them if those elements are contained in excess. It was found from a result of the study conducted by the inventor that nickel can be fully removed by annealing it in an atmosphere of hydrogen chloride, various methane chlorides (CH3C1 etc.), various ethane chlorides (C2H3Cl3, etc.) and various ethylene chlorides (C2HCl3, etc.) at 400 to 650xc2x0 C. It was found that the concentration of nickel, iron, cobalt, platinum or palladium within the silicon film of the present invention was preferably set at 1xc3x971015 cmxe2x88x923 to 1 atomic %, or the minimum concentration of nickel, iron, cobalt, platinum and palladium was more preferably 1xc3x971015 cmxe2x88x923 to 1xc3x971019 cmxe2x88x923 with a measured value of SIMS. Crystallization does not fully advance at a concentration less than this range and, conversely, characteristics and reliability are degraded when the concentration exceeds this range.
Various physical and chemical methods may be used for forming film nickel, iron, cobalt, platinum or palladium. They are, for example, those methods which require vacuum equipment such as a vacuum deposition method, sputtering method and CVD method and those methods which may be performed in air such as a spin coating method, dip method (application method), doctor blade method, screen printing method and spray thermal decomposition method.
Even though the spin coating method and dip method require no particular facility, they allow production of a film having a homogeneous film thickness and minute control of the concentration. As a solution used for these means, whatever acetates, nitrates or various carboxylic acid salts or other organic acid salts of nickel, iron, cobalt, platinum or palladium dissolve or diffuse in water, various alcohols (low and high grades) and petroleums (saturated hydrocarbons or unsaturated hydrocarbons) may be used.
In such a case, however, there has been the possibility that oxygen and carbon contained in those salts might diffuse within the silicon film, degrading the semiconductor characteristics. However, as a result of research advanced by a thermobalance method and differential thermal analysis, it was confirmed that they are decomposed into oxide or a simple substance in an adequate material at a temperature of less than 450xc2x0 C., and that they would not diffuse into the silicon film thereafter. When such low order substances as the acetate and nitrate were heated under a reduced atmosphere such as a nitrogen atmosphere, they decomposed at less than 400xc2x0 C. and became a simple metallic substance. Similarly, it was observed that when they were heated in an oxygen atmosphere, oxide was initially produced and eventually become a metallic simple substance as oxygen desorbed at higher temperatures.
The manner in which the foregoing and other objects of the present invention are accomplished will become apparent from the accompanying specification and claims, taken together with the drawings.